Delayed luteal regression in ewes immunized against oxytocin.

Active immunization of sheep against oxytocin prolonged the luteal phase of the oestrous cycle, as judged by oestrous behaviour and circulating progesterone concentrations. Mean cycle length was extended by 3.7 days. The treatment resulted in a 10-fold increase in circulating oxytocin concentrations. Antisera produced were specific for oxytocin; cross-reactions with vasotocin, arginine vasopressin, and hypothalamic releasing factors were low. Cerebrospinal fluid contained low levels of antibodies directed against oxytocin. This finding supports the hypothesis that the luteolytic action of oestradiol-17 beta in sheep may be mediated through a stimulatory effect on the endometrial oxytocin receptor concentration.